1. Field of the Invention
The present invention relates to an opportunity driven multiple access (ODMA) system and, particularly, to a data transmission method for the ODMA system which controls transmission power according to data priority in the ODMA system.
2. Description of the Related Art
Recently, interest and demand for mobile communication services have dramatically increased. Various mobile communication services are provided to users having mobile communication terminals. Some mobile communication service include multimedia services such as data and motion picture information, as well as voice information. Accordingly, much research in the area of communication technologies and mobile communication terminals is being performed.
Generally, a cellular system divides a large service area into a plurality of small cells, covered by base stations for managing the cells. The base stations are installed in respective cells not to interfere with each other. The base stations are disposed at a distance so that radio waves do not interfere. Same frequency is not used for adjacent cells.
Since respective base stations are connected to a central control station and to a switching station, when a mobile terminal crosses over a cell boundary, this cross over can be detected and speech status can be monitored by transmitting/receiving connection control information of mobile terminals in other cells, as well as the mobile terminal in its cell. When the mobile terminal moves to another cell, the active traffic channel is changed to a frequency channel corresponding to the new cell, to compete a hand-off.
A cell centering around a base station can be divided into a high speed data area and a low speed data area. When a mobile terminal in the cell moves further away from the base station and reaches the cell boundary area, a lot of path loss occurs. Therefore, when the mobile terminal has moved at a certain distance from the base station, it is difficult to transmit the data with high speed and the speech quality is lowered due to the path loss.
FIG. 1 is a view showing a mobile communication system according to the conventional art, and illustrating a method for providing a mobile terminal near the cell boundary with the data service in a cellular environment.
Dotted line A shows the data transmission in a general cellular mobile communication system, wherein the base station transmits the data directly to an object terminal. Since the mobile terminal in the cell boundary area is far away from the base station, much transmission loss is generated during data transmission, and thereby data transmission rate is lowered and error rate is increased. Therefore, it is difficult to provide high speed data service to a mobile terminal which is far away from the base station, and accordingly, only low speed data service can be provided.
In addition, when the mobile terminal receives the data, the terminal requires a received power more than a certain degree. Accordingly, when the base station transmits the data to the mobile terminal which is far away from the base station, the base station should use a strong transmission power to correspond the received power. Unfortunately, a strong transmission power may interfere with a mobile terminal connected to an adjacent base station, and may affect the speech quality.
Since resources are limited in a radio environment, minimum transmission power should be used, when possible. However, when the mobile terminal requiring multimedia service at high receiving rate and low error rate moves to the cell boundary area, the interference for the other users in the cell may be increased and the speech quality may be lowered. Therefore, a more efficient and effective method for managing radio link resources is needed.
In order to solve the problems of general cellular system, many researches for providing multimedia service of high speed in an IMT-2000 system are being performed, for example, an opportunity driven multiple access (ODMA) system has been introduced.
The ODMA system is a system made by applying concepts of ad-hoc network to a mobile communication system, and transmits the data through various hops for increasing efficiency of universal mobile telecommunications system (UMTS), and thereby, constructing a larger high speed data service area. The ODMA is a protocol for relaying communication information between terminals so that the data communication at high speeds can be maintained even in the cell boundary area.
Referring to FIG. 1, line B illustrates a method for transmitting data in an ODMA system according to the conventional art. As shown therein, when the base station transmits the data to the mobile terminal in the cell boundary area, the base station transmits the data through a plurality of hops. In the ODMA system, the high speed data service area can be expanded with a small transmission power, and therefore, the ODMA system is effective in view of power consumption comparing to the conventional cellular system transmitting the data directly to the distant mobile terminal.
Generally, the multimedia data transmitted between the mobile terminals has various attributes such as delay-sensitive voice and picture communication requiring real-time transmission, and file transmission which does not require the real-time transmission.
However, the conventional ODMA system uses uniform transmission power regardless of the delay sensitivity or the traffic attributes of the data transmitted/received between terminals, and thereby it is difficult to provide multimedia data services having various attributes.